Rising labor costs and demands for more time and cost efficient construction have made it desirable to construct building components and modules off-site at specialized fabrication facilities. With wood frame structures, especially prefabricated residential structures, there are great economies to be realized by providing equipment that can automatically measure and cut the multiple different lumber components utilized in wall panels, roof trusses, floor trusses, and other prefabricated structures. Where significant quantity of a particular structural element, such as a roof trusses, is needed, the use of such automated equipment can greatly decrease construction time and lower cost. The economies of this approach are very appealing for custom structural designs. For wood structures where the framing is constructed on site, precutting and marking lumber off site can create a kit design minimizing measuring, sawing, and specialized labor on site. This can result in faster construction as well as minimized cost.
The use of prefabricated trusses or panels also minimizes construction delays due to the interference of bad weather. Trusses and panels can be constructed in a controlled indoor environment.
Prefabricated roof trusses in particular, generally include multiple pieces of lumber that must be precision cut to specific lengths as well as having precision mitered ends to form tight fitting joints. As depicted in FIG. 1, a typical roof truss includes two top chords TC, a bottom chord BC, several webs WB and may also include wedges WD and overhangs O. Many of these pieces require a preparation of mitered cuts at the ends of the lumber pieces. Many of the pieces will require multiple mitered cuts on an end. Truss plates with teeth are typically utilized to securely make the connection. For a truss to achieve its maximum structural integrity and strength the joints between the various wooden parts should be tight fitting. Thus precision cutting of truss members is quite important to creating a truss that meets engineering standards.
Thus, the process for cutting and mitering truss members, in many circumstances, has been automated for improved precision.
Wood, however, is a natural product and is subject to certain imperfections. Lumber is sawed and planed to size and shape and is also often kiln dried to achieve a desired level of moisture content. As lumber is dried it may acquire a certain degree of warpage or crookedness.
In many or most applications, the length of the cut board with mitered ends is critical. Typically, automated cutting systems make no allowance at all to adjust for warpage or crookedness of lumber members and the length of the board after the mitered cut will often deviate significantly from the specified length such that the board is not usable. This occurs because the miter saw cuts in a plane at an angle with respect to the axis of the board and if the board is crooked upwardly or downwardly, the board will be cut in a different location on the saw blade plane and be longer or shorter than intended. Some automated cutting systems compensate for crooked lumber by forcing crooked lumber pieces to a straight orientation before cuts are made. This is commonly accomplished by the application of force through hydraulic or pneumatic pistons. The problem with this approach is that when the straightening force is released the lumber member will generally spring back to its pre-straightened status. The precisely made cut is then dislocated from its original position and reduces the precision with which trusses assembled from the warped lumber members can be made.
In addition, heavier lumber members such as 2×12 members are very resistant to being forced to a straight orientation. The force required to straighten heavy lumber may exceed the capacity of the equipment to apply it or the lumber may split, crack or break.
The effect of lumber member crookedness on the length of the cut lumber member is limited when cuts are made to the lumber member at or near to ninety-degree angle with respect to the length of the member. However, when mitered cuts are made, lumber member crookedness alters the length of the finished piece significantly. At a forty-five degree cut crookedness essentially alters the finished length in a one to one ratio. As the miter angle is farther from ninety degrees the variation in length becomes larger than the amount of crookedness at a greater rate.
Thus the frame lumber prefabrication industry would benefit from a system to compensate for crooked lumber in automated measuring, cutting and lumber handling equipment.